Halogenation of Electron‐Deficient Vicinal Substituted Alkenes: Regio‐ and Stereoselectivity

Abstract

The halogenation of electron‐deficient vicinal substituted alkenes leads mainly to the mixture of regio‐ and stereoisomers of monohalogenated derivatives. Their ratio depends on the stability of the intermediate anionic complex and is determined by properties of both electron‐withdrawing groups.

Chemoselective Bromination of Dienoates

“…Other convenient brominating reagents such as 1,2-dipyridiniumditribromide-ethane (DPTBE) or HBr in the presence of oxone were tested with ethyl sorbate. In contrast to the experiments using molecular bromine, in these cases the formation of ethyl 4,5-dibromohexenoate 6b as a principal reaction product was accompanied by the significant formation of isomeric 2,3-dibromoenoate 7b: its amounts were reached by 33 % (Table 1, entries 11,12).…”

“…The syntheses of bromoenoates described so far are based on the commonly used methods, namely bromination -dehydrobromination of parent compounds by molecular bromine or different organobromine compounds as halogenating reagents. [11] As a continuation of our research in this field, [12] here we report an efficient synthetic procedure for the bromination of dienoates. Depending on the reaction conditions, we prepared mono-and dibromodienoates and their precursors in good to excellent chemo-and stereoselectivity.…”

“…The dibromoderivatives 6a-c and 7b,c were isolated in pure form by column chromatography on Silica gel. (12). We had troubles obtaining suitable elemental analysis for compounds 7b,c due to their low stability and propencity to loss HBr.…”

Pull‐Pull Alkenes in the Aza‐Michael Reaction

“…It should be noted that in one of the first articles the structure of dehydrobromination product was erroneously determined as 3‐bromo‐4,4,4‐trifluorocrotonate [51] . The complete and confident proof for the structure of this derivative was published recently [61] …”

“…[51] The complete and confident proof for the structure of this derivative was published recently. [61] To test whether the starting bromoenoate 52 can react as Michael acceptor, this substrate was treated with secondary amine under classical and hyperbaric conditions. It was found that the ester Z-52 a reacted with morpholine in ethanol under reflux to afford βamino ester 53 as a sole reaction product (Scheme 21).…”